The present invention relates generally to a device and method for injecting bridging materials and cementitious mud downhole for the purpose of controlling severe lost circulation and, more particularly, to a device and method for emplacing a quick-setting cement downhole while ensuring that premature setting does not occur inside the drill pipe. This invention is useful for any downhole or drilling operation where the problem of lost circulation is likely is occur, e.g. oil and gas drilling, geothermal drilling, coring operations, and mineral exploration.
Lost circulation is the phenomenon where circulating drilling fluid is lost to fractures or pores in the rock formation rather than returning to the surface through the wellbore annulus, as it does during normal drilling. In a wellbore, drilling fluid, such as cementitious mud, is pumped downhole and circulates to the surface to cool the bit, to carry rock chips out of the borehole, and in some cases to control the well; when lost circulation occurs, this fluid is lost to the rock formation due to an incompetent or permeable rock formation (characterized by a porous matrix, fractures, vugs, or caverns) which does not have adequate physical integrity or pore-fluid to support the hydrostatic pressure inside the wellbore.
Although drilling can continue under lost circulation conditions, it is generally imperative that the fluid loss be stopped as soon as possible after it is discovered for various reasons: the loss of the drilling fluid itself to the formation is expensive; changes in the rock formation being drilled cannot be easily detected if rock chips are not circulated out of the wellbore; rock chips lost to the formation can flow back into the wellbore when drilling stops, thus sticking to the drillstring in the hole; control of the well may be difficult or impossible if a high-pressure zone is encountered with the wellbore only partially filled with drilling fluid; drilling fluid invasion of the surrounding rock formation alters in-situ conditions and therefore affects the logging response of the formation; freshwater aquifers associated with loss zones can be contaminated by drilling mud and connate fluids (fluids trapped in the sediment and/or rock) produced at different wellbore intervals; and loss zones not treated during the drilling phase can cause casing cement to be lost to the open formation during completion operations, resulting in a poor or incomplete bond between the casing and the rock formation and requiring expensive remedial action to prevent inter-interval flow and (in geothermal wells) possible casing collapse when the well is put on production.
Lost circulation is a major problem in oil and gas well drilling and other types of exploration with the advent of exploration in deeper, more highly fractured producing formations; however, lost circulation problems tend to be more severe in geothermal drilling than in other types of drilling because of the highly fractured and underpressured nature of many geothermal formations. Bridging materials (i.e. the particles added to drilling mud to form a bridge or a plug across a fracture) used as drilling mud additives for lost circulation control in oil and gas drilling are ineffective in plugging large fracture apertures, particularly under high-temperature conditions. Therefore, the standard lost circulation treatment in geothermal drilling is to fill the loss zone surrounding the wellbore with cement, which is both expensive and time-consuming due to the necessity of waiting for the cement to harden and then drilling through the cemented zone to reach new rock formation.
In geothermal drilling, lost circulation is typically the most costly problem routinely encountered. In mature geothermal areas, lost circulation costs represent an average of 10% of the total well costs, and in exploratory wells and developing fields, lost circulation costs often account for over 20% of total well costs.
Various methods and apparatus are known for delivering materials into the wellbore and/or for providing fluid access to the wellbore annulus, but most do not address the problem of lost circulation control.
U.S. Pat. No. 3,799,278 to D. L. Oliver described a downhole tool for providing fluid access to the wellbore annulus through the side of the drill pipe using a dropped dart or wireline in order to restore drilling mud circulation if the drill bit nozzle becomes clogged during operation. U.S. Pat. No. 4,072,166 to Tiraspolsky et al. describes a downhole tool for providing fluid access from the wellbore annulus to the drill pipe interior through the side of the pipe using the pressure drop of the flowing fluid to operate a valve, the purpose of the invention being to ensure the axial flow of fluid injected into the drill pipe during drilling while allowing interruption of the axial continuity and connecting the interior of the pipe directly with the exterior annulus space when the injection is broken off or when the flow descends below a minimum value.
U.S. Pat. No. 4,645,006 to Tinsley describes a downhole device for providing access to the wellbore annulus through the side of the drill pipe using the drill pipe internal pressure acting on a dropped actuator to open a sliding access valve, in order to restore the circulation of drilling mud if the drill bit nozzle becomes clogged during operation. U.S. Pat. No. 4,823,890 to Lang describes a reverse circulation drill bit and associated apparatus in a permanent concentric tubing arrangement for directing the flow of drilling fluids through the bit in a reverse circulation mode.
Thus, both the direct costs, and the unknown costs associated with possible contamination of freshwater aquifers, as well as other problems related to lost circulation control indicate an existing need for a system providing major-fracture fluid loss control. More particularly, there is an existing need for technology to plug major-fracture loss zones.
In addition to cost considerations, when the maximum thickness of the loss-zone fractures is greater than the diameter of the drill bit nozzles, it is not possible to plug the loss-zone with drilling mud additives without also plugging the bit nozzles. In such cases, it is necessary to use a material that either solidifies after it flows through the bit or is emplaced downhole after first removing the bit. In geothermal drilling, various cement formulations are pumped downhole for plugging major-fracture loss zones. While these cement treatments are generally effective in stopping fluid loss, they are expensive in both the quantity (hundreds of cubic feet) of cement required and the long waiting time (8-12 hours) for the cement to set before drilling can resume.
A new class of cementitious material is known as cementitious mud, which consists of bentonite drilling mud with added constituents for turning it into solid form, usually including an accelerator material for controlling the setting time. The formulations are developed to provide rapid-setting, temperature-driven, cements in which significant compressive strengths may be developed within short times. As an example, a cement formulated by mixing conventional bentonite mud with ammonium polyphosphate, borax, and magnesium oxide has been developed which attains significant compressive strength in less than two hours when sufficient concentrations of the magnesium oxide accelerator are used; the setting time decreases with temperature, and the material expands approximately 15% upon setting.
Even with the potential benefits derived from the use of these muds for plugging purposes, there is a problem with lack of control over the setting process to ensure that the fluid will not set up inside the drill pipe during field application. Thus, there is an existing need for an alternative emplacement technique for more effectively and economically plugging loss zones dominated by large fractures, vugs, and caverns. There is also an existing need for an alternative emplacement system that provides more control over the setting process in the hole so that the cementitious mud will not set up inside the drill pipe during field operation.
In an effort to find alternative materials for more effectively plugging major-fracture loss zones, cementitious muds with an encapsulated accelerator have been developed. Specifically, the accelerator, typically the magnesium oxide additive, is encapsulated with an inert material that is sheared off by fluid action at the bit nozzles. The inert material used for the encapsulant for the accelerator may be one of many materials. In this technique, the cementitious mud is mixed at the surface and pumped downhole, but since the accelerator is shielded from the other cement constituents by the inert encapsulant, the cement does not harden in the drill pipe regardless of the time required for pumping. As the cement flows through the nozzles, the encapsulant is sheared off, exposing the accelerator and initiating the cement setting process. The chemical setting reaction is then further accelerated as the cementitious mud flows into the high temperature formation. However, questions exist as to the timing and reliability of the encapsulation technique.
There is an existing need for an alternative system for emplacing cementitious mud downhole in case the encapsulation technique is unworkable, either consistently or at some proven parameters.
Known apparatus and methods for delivering plugging materials to the wellbore that do address the problem of lost circulation are subject to the necessity of avoiding premature set-up of the plugging material and the problems associated therewith. U.S. Pat. No. 4,378,050 to Tatevosian et al. describes a downhole tool for delivering a pre-mixed plugging material in a container to the bottom of a drill pipe and injecting it into the wellbore through the bit using a displacing agent (mechanical, fluid, or gas) to force the plugging material into the bit, with the goal of plugging a lost circulation zone. U.S. Pat. No. 4,842,066 to Galiakbarov et al. describes a downhole device for injecting a single stream of pre-mixed cement slurry downhole through the drill pipe to the location of a lost circulation zone in order to accomplish downhole separation of the components of a single fluid stream of cement slurry into a solid and liquid phase, with the purpose of plugging the lost circulation zone.
There is an existing need for a method and corresponding system for quickly and economically plugging lost circulation zones without requiring pulling or tripping the bit.
There is also an existing need for a method and corresponding system to allow the components of a two-component plugging material, such as cementitious mud, to be placed downhole simultaneously but separately, without mixing the components prior to emplacement in the wellbore, for lost circulation control.